Microwave grain monitors employing analog circuits for determining crop yield and the moisture content of grain are well known in the art. Typically, the prior art systems comprise an RF frequency source for applying a constant power microwave measurement signal in the range of 1 to 10 GHz to a transmit antenna disposed on one side of a sample region containing a grain sample, a receive antenna disposed on the opposite side of the sample region, analog circuits connected to the receive antenna and the signal source for producing indications of the phase shift and attenuation imposed on the measurement signal during its passage through the grain sample, and a computer responsive to the indications of the attenuation and phase shift for determining the moisture content and bulk grain density of the grain. Once the moisture content and bulk density have been determined, crop yield may be determined from these values and from a measurement of grain mass flow through the sample region, the flow measurement typically being made by a Doppler microwave transceiver. A typical microwave system for measuring the moisture content of grain is described by King et al. in a technical paper entitled Microwave Moisture Measurement of Grains, published in the IEEE Transactions on Instrumentation and Measurement, Vol. 41, No.1, February 1992, pp. 111-115.
Prior art systems employ analog receiver circuits including expensive linear receiver amplifiers, detectors and baseband amplifiers to detect the phase shift and attenuation imparted to the measurement signal as it passes through a grain sample. Such systems require a fixed RF power from the transmitter sufficient to overcome all losses and still produce a good signal output. The receiver must be linear from the noise floor to maximum input power so that it will not distort the test results. The dynamic ranges of the prior systems have had to include no signal input to maximum transmit input signals resulting in a dynamic range of from 60 to 80 dB.
Most prior art systems measure both the attenuation and the phase shift imparted to a measurement signal in passing through the grain, and moisture content is computed from these values using an experimentally developed formula. The above-referenced copending application discloses an improved grain monitor wherein moisture content is determined by measuring only the attenuation. Phase shift is primarily a function of bulk grain density whereas attenuation is primarily a function of moisture content. The referenced application teaches that, by maintaining a substantially constant bulk grain density in the sampling region, moisture content may be determined by measuring the attenuation only, and with only slightly less accuracy (1.2% error versus 1.0% error) than when both phase shift and attenuation are measured. While this system represents an improvement over the prior art in that it avoids the use of the circuits for determining phase shift, it still employs relatively expensive linear analog receiver circuits for measuring the attenuation.